What is a semaphore and how is it used for process synchronization?

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What is a semaphore and how is it used for process synchronization?

A semaphore is a synchronization primitive used to control access to shared resources and to coordinate the order in which processes run. It can be binary, acting like a lock with a value of 0 or 1 to enforce mutual exclusion, or counting, which tracks how many instances of a resource are available. The mechanism revolves around two operations: wait (often called P or down) and signal (often called V or up). When a process performs a wait, it attempts to decrease the semaphore’s value; if the value would go negative, the process blocks until another process signals. When a process performs a signal, it increases the value and, if any processes are waiting, wakes one of them. This lets multiple processes coordinate access to a shared resource without two processes entering the critical section at the same time (in the binary case) and allows up to a limit of concurrent accesses (in the counting case). Semaphores are about synchronization and resource management, not about exchanging messages, managing memory allocation, or measuring CPU usage.

A semaphore is a synchronization primitive used to control access to shared resources and to coordinate the order in which processes run. It can be binary, acting like a lock with a value of 0 or 1 to enforce mutual exclusion, or counting, which tracks how many instances of a resource are available. The mechanism revolves around two operations: wait (often called P or down) and signal (often called V or up). When a process performs a wait, it attempts to decrease the semaphore’s value; if the value would go negative, the process blocks until another process signals. When a process performs a signal, it increases the value and, if any processes are waiting, wakes one of them. This lets multiple processes coordinate access to a shared resource without two processes entering the critical section at the same time (in the binary case) and allows up to a limit of concurrent accesses (in the counting case). Semaphores are about synchronization and resource management, not about exchanging messages, managing memory allocation, or measuring CPU usage.

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